This star is located about
15.9 light-years (ly) away from our Sun, Sol, in the northwestern
corner of (10:11:22.1+49:27:15.2, ICRS 2000.0) of Constellation
Ursa
Major, the Great Bear, which also encompasses the Big
Dipper or Plow (Plough) -- southwest of
Merak
(Beta Ursae Majoris), north of
Tania
Australis and Borealis (Mu and
Lambda
Ursae Majoris), and
east of
Talitha
(Iota Ursae Majoris). The star was listed by
Stephen
Groombridge (1755-1834), whose "A Catalog of Circumpolar
Stars, Reduced to January 1, 1810" [see page 54] was published
posthumously in 1838. Although Groombridge 1618 is brighter and
more orange or hotter at the surface than most nearby flare stars
(which are typically classified as red dwarfs), it has also been
relatively less active.

Astronomers have identified Groombridge 1618
as a target for NASA's
optical SIM
mission,
now indefinitely delayed.

Due to Groombridge 1618's proximity to Sol, the system has been
an object of high interest among astronomers. The star has been
selected as
"Tier 1"
target stars for NASA's optical
Space
Interferometry Mission (SIM). The mission will attempt to
detect planets as small as three Earth-masses within two AUs of
each star. Although some
summary
system information and images of
Groombridge
1618 may still be available from the
SIM
Teams, the SIM project manager announced on November 8, 2010
that the mission was indefinitely postponed due to withdrawal of NASA
funding.

The Star

Groombridge 1618 is a orange-red main sequence dwarf star of
spectral and luminosity type K7 Vne, that has been classed
as orange as K5 by the
SIMBAD
Astronomical Database (which may be using an older
reference) and as red as M0. This star has about 64 percent
of Sol's mass
(RECONS
estimate), 59 to 68 percent of its diameter
(Pasinetti-Fracassini
et al, 2001), and about 4.6 percent of its visual luminosity
and 13.3 percent of its theoretical bolometric
luminosity, correcting for infrared output
(NASA
Star and Exoplanet Database, derived using exponential formula from
Kenneth
R. Lang, 1980).
The star appears to be about 191 percent as enriched as Sol
with elements heavier than hydrogen ("metallicity"), based on
its abundance of iron
(Cayrel
de Strobel et al, 1991, page 290). Groombridge 1618
is chromospherically active with star spots
(Vassiliki
Tsikoudi, 1989). Its status as a flare star and its
average equatorial rotational velocity speed of 1.7 to 2.1
(+/- 0.5 to 0.8) km (1.1 to 1.3 miles) per second
(Marcy
and Chen, 1992; page 553) suggest that it may be a
relatively youthful star that is over a billion years old.
It is a New Suspected Variable star designated
NSV 4765. Useful star catalogue numbers for Groombridge 1618
include: Gl 380, Hip 49908, HD 88230, BD+50 1725, G 196-9,
SAO 43223, LHS 280, LTT 12732, and LFT 696.

Since Groombridge 1618 is sort of like a distant cousin
to Sol, some speculate whether it might just be bright enough to
support Earth-type life on a planet lucky enough to orbit in its
water zone. Estimates provided by the
NASA
Star and Exoplanet Database indicate that the inner edge of Groombridge
1618's habitable zone could be located around 0.354 AU from
the star, while the outer edge edge lies around 0.691 AUs. The
distance from the star where an Earth-type planet could have liquid
water on its surface is centered around 0.523 AU -- between the
orbital distances of Mercury and
Venus in the
Solar System.
At that distance from the star and assuming that it has 0.64 Solar-mass,
such a planet would have an orbital period of nearly 173 days
(or close to half an Earth year).

Hunt for Substellar Companions

No stellar or brown dwarf companion of at least seven
Jupiter-masses with an orbital period of less than
four years has been detected around Groombridge 1618
(Marcy
and Benitz, 1989). The star was included in a subsequent imaging search
with adaptive optics found that "with 90% confidence no more than 50% of stars
like those in our survey have a 5 MJup or more massive planet orbiting between
30 and 94 AU, no more than 15% have a 10 MJup planet orbiting between 22 and
100 AU, and no more than 25% have a 20 MJup object orbiting between 8 and 100
AU" (Heinze et al,
2010).

Life Around a Flare Star

Many dim, red (M) and some of the dimmer orange-red (K) dwarf
stars exhibit unusually violent flare activity for their size
and brightness. These flare stars are actually common because
red dwarfs make up more than half of all stars in our galaxy.
Although flares do occur on the Sun every so often, the amount of
energy released in a Solar flare is small compared to the total amount
of energy that Sol produces. However, a flare the size of a solar
flare occurring on a orange-red dwarf star (such as Groombridge 1618)
that normally has less than five percent of the luminosity of the
Sun would be quite noticeable.

Groombridge 1618 is a flare star, like UV
Ceti (Luyten
726-8 B) shown flaring at left. UV Ceti is an extreme
example of a flare star that can boost its brightness by
five times in less than a minute, then fall somewhat slower
back down to normal luminosity within two or three
minutes before flaring suddenly again after several hours.

Flare stars erupt sporadically, with successive flares spaced anywhere
from an hour to a few days apart. A flare only takes a few minutes to
reach peak brightness, and more than one flare can occur at a time.
Moreover, in addition to bursts of light and radio waves, flares on dim
red dwarfs may emit up to 10,000 times as many X-rays as a
comparably-sized Solar flare on our own Sun, and so flares would be
lethal to Earth-type life on planets near the flare star. Hence,
Earth-type life around flare stars may be unlikely because their
planets must be located very close to dim orange-red dwarfs to be
warmed sufficiently by star light to have liquid water (about 0.22 AU
for Groombridge 1618), which makes flares even more dangerous around
such stars. In any case, the light emitted by late orange-red dwarfs
may be too red in color for Earth-type plant life to perform
photosynthesis efficiently.

Closest Neighbors

The following star systems are located within 10
light-years of Groombridge 1618.

Constellation Ursa Major is only visible from the northern hemisphere.
The seven stars of the Big Dipper in this constellation are famous as
the traveller's guide to Polaris, the North Star. For more
information about the stars and objects in this constellation, go to
Christine Kronberg's
Ursa Major.
For another illustration, see David Haworth's
Ursa
Major.

For more information about stars including spectral and luminosity
class codes, go to ChView's webpage on
The Stars of
the Milky Way.